See the small white forehead patch of the male Collared flycatcher in the photo? They are getting smaller and smaller.
Scientists from the Eötvös Loránd University in Hungary studied a population of these collared flycatchers for 15 years between 1990 and 2004, and found that the size of the white patches on males in the population are on average, getting smaller. While the specific reason for this reduction in forehead patch size in the population is still a mystery, the scientists suggests that the change in environment made it more difficult for birds with more “flashy” display to survive over the winter. For more details and discussion, see their publication.
The collared flycatcher breeds in southeast Europe and migrates to sub Sahara Africa during the winter.
For more recent research on the collared flycatcher, see this publication.